This invention relates generally to radar processing systems, and more particularly to pulse compression systems yielding low range-time sidelobes.
It is well known in the art of pulse radar systems that in order to obtain a good detection capability against a background of the noise, a pulse with a large energy content must be transmitted. This large energy content may be obtained by either transmitting a pulse with a large peak power and/or with a long pulse duration. If the pulse width is limited to small values because of the desire to obtain good range accuracy or resolution, the required energy pulse must be obtained with a large peak power. However, in many applications it is not possible to obtain a peak power as large as one might desire because of voltage-peak limitations somewhere in the system. In such peak-power-limited radar systems, the required energy can be obtained only by transmitting a longer pulse.
It is now recognized that the range resolution is not governed by the pulse length but by the overall transmitted bandwidth. Thus, by modulating the carrier within the transmitted pulse length and thereby increasing the bandwidth, the range resolution may be improved with no reduction in mean transmitted power.
Radars using such modulated carrier pulses to increase the bandwidth of the signal are referred to as pulse compression systems. The use of such pulse compression techniques permit the transmitted pulse to be made as long as desired while retaining an optimum range resolution.
The modulation or coding utilized to increase the bandwidth of the transmitted pulse may be either phase or frequency coding. In the compression process, a long coded pulse with an increased bandwidth B much greater than the reciprocal of the pulse length due to the frequency or phase coding is transmitted and echo returns therefrom are decoded to form short pulses with a duration equal to 1/B.
It has been found that the use of the Frank phase code (R. L. Frank, "Polyphase Codes With Good Non-Periodic Correlation Properties", I.E.E.E. Transactions IT-9, 43-35, January 1963) is especially advantageous because it has excellent doppler tolerance and very low range-time sidelobes (the highest sidelobe is approximately p.pi..sup.2 down from the mainlobe, where p is the pulse compression ratio). Such phase codes are generated by transmitting a phase for a time interval t=1/B, changing the phase for the next time interval 1/B, etc.
Typical prior art Frank code pulse compression systems employed phase coded transmitted signals and utilized pipeline compressors for the matched filtering process. However, because there are abrupt zero to .pi. phase shifts from element to element in certain portions of the Frank code, problems arose during the reception process. In particular, when a receiver was utilized with a bandwidth equal to the reciprocal of the length of one code element (the compressed pulse length), then the abrupt phase shifts noted above caused amplitude modulation on the signals passing through the receiver. The amplitude modulation was due to the fact that the period of time required for a circuit such as a receiver to build up to a given amplitude is inversely proportional to the bandwidth of the circuit. A zero to .pi. phase shift requires the signal amplitude of the receiver to go to zero and then a build up to the same signal amplitude but with a.pi. phase. Thus, when abrupt phase changes of .pi.radians occur in a code, as they do in the mid-section of the Frank code, the band limited receiver must de-ring and re-ring with the amplitude of the response going through zero with a non-infinite slope. Clearly, such amplitude modulation will increase the range-time sidelobes of the response.
Additionally, the pipeline compressors used in such prior art systems employed a tapped delay line with N taps each separated by the time duration of a code element, a series of phase shifters, one phase shifter connected to each of the N taps, and an N-way adder to add the outputs from the N phase shifters. Accordingly, such pipeline compressors used many elements and were inefficient. Moreover, the phase shifting elements utilized therein included L and C reactances which would cause circuit ringing thereby yielding higher range-time sidelobes.
A recent invention by Lewis and Kretschmer, "Low Sidelobe Pulse Compressor", patent application Ser. No. 230,984, solved this amplitude modulation problem and the inefficient compressor problem by recognizing that the Frank phase code may be derived from sampling an in-phase I and a quadrature Q detected step approximation to a linear frequency modulated waveform. They also recognized that the Frank phase code was the complex conjugate of the steering weights of a Fast Fourier Transform (FFT) circuit. In the above referenced application, Lewis and Kretschmer disclosed a compressor system that transmitted a step approximation to a linear frequency modulated waveform, a receiving circuit, means for I and Q detecting the echo signals, means for sampling these signals once per compressed pulse length, means for converting the samples to digital words, means for passing these digital words through a digital FFT, means for differentially delaying the output frequencies obtained therefrom in order to make them simultaneous, and means for adding the simultaneous frequencies to form the compressed pulse. Each frequency in the transmitted waveform lasted for the square root of the pulse compression ratio longer than the duration of the compressed pulse. Thus, the receiver amplifier did not modulate the echo significantly. Moreover, the FFT replacement for the pipeline compressor reduced the number of operations that had to be performed and significantly increased the efficiency.
The digital Fourier transform circuit utilized in that digital system sampled the incoming echo signal at certain discrete times and then compared these samples of the echo with the transmitted waveform and produced an output which was the autocorrelation function of the transmitted waveform. This autocorrelation function had a maximum value when the sampling was begun simultaneously with the occurrence of the leading edge of the incoming target-echo signal. However, when the target-echo leading edge arrived half way between samples, then the maximum of the autocorrelation function was significantly reduced. The target echo signal energy was essentially spread over two or more sampling periods or range cells resulting in a flattened maximum mainlobe with a low mainlobe to sidelobe ratio, e.g., the compressed pulse height was reduced and the compressed pulse length was increased. Thus, such a half sampling period error yielded a significant range resolution loss and a decrease in radar sensitivity.
Such spreading of the mainlobe is especially detrimental in radar systems because it allows large cross-section targets with large sidelobes to effectively mask smaller targets, i.e., an aircraft with a large cross-section could effectively mask a missile of much smaller cross-section. Accordingly, this flattening of the mainlobe relative to the sidelobes is of prime concern in the reflected signal processing art.